It is well known in the art that cracking of hydrocarbons with silica or silica/alumina base catalysts produces coke on such catalysts which can be burned off. One of the problems arising in the cracking step consists in the detrimental effects brought about by such metals as nickel, vanadium, and iron contained in many hydrocarbon feedstocks. These metals cause production of hydrogen and coke. A problem arising in the catalyst regeneration step resides in the fact that the ratio of carbon dioxide to carbon monoxide normally decreases with increasing temperature. Therefore, although high temperatures are a desirable process parameter, it is not feasible to increase the temperature since this would lead to a further decrease in an already low carbon dioxide to carbon monoxide ratio in the off gases.
It has been claimed in the art that antimony, bismuth and manganese added to a cracking catalyst increased the carbon dioxide to carbon monoxide ratio. The art gives examples utilizing antimony and bismuth. There is a continuing interest in increasing the CO.sub.2 /CO volume ratio in a catalyst regeneration step.